Type 2 diabetes mellitus (T2DM) affects 7.8% of US adults. Women with T2DM are three times more likely to have depression than their counterparts without T2DM. The comorbidity between depression and T2DM is a concern because it is associated with worse glycemic control and poor diabetes self-management. Studies have shown that exercise improves glycemic control among individuals with T2DM and improves depressive symptoms among individuals with major depressive disorder. Exercise might then be a parsimonious treatment approach for women with comorbid T2DM and depression by improving both conditions. One limitation of exercise as an effective treatment modality for T2DM and depression is that patients with this comorbidity report difficulty initiating and maintaining exercise, making them especially resistant to exercise recommendations. Exercise interventions should be theoretically-based and include strategies to address the motivational challenges of this population. We aim to develop and pilot test a novel exercise intervention that is based on principles of behavioral theory and incorporates strategies of behavioral activation treatment for depression (BA). In the developmental phase, BA strategies will be integrated into an exercise class intervention to be led by an exercise instruction professional. A pre-pilot exercise class (n=10) will then be conducted to examine treatment acceptability and feasibility. Once intervention materials are refined, sedentary, overweight women with inadequately controlled T2DM and depression (n=60) will be randomized to either the 24-week BA-enhanced exercise condition or the usual care condition. The primary outcomes will be feasibility (adherence, treatment acceptability, exercise enjoyment) and initial efficacy (depression score and glycosylated hemoglobin) at 3-, 6- and 12-months following randomization. Secondary outcomes measured at baseline, 3-, 6- and 12-months, include physical activity, diet, exercise self-efficacy, fitness, blood pressure, body weight, waist circumference and quality of life. Products of this study include a treatment manual and preliminary data for a larger randomized controlled trial. PUBLIC HEALTH RELEVANCE: Women with type 2 diabetes mellitus (T2DM) are at heightened risk for depression, and the combination of T2DM and depression is associated with worse health outcomes. Exercise has been shown to improve diabetes management as well as depression, but has not been evaluated as a treatment for women with both T2DM and depression. This population is often more sedentary and reports greater barriers to exercise. The proposed work aims to develop a unique exercise intervention that addresses exercise barriers and improves diabetes management and depression in women with T2DM and depression.